This invention relates to the use of low dielectric constant, low moisture uptake, thermally stable polyimides and copolyimides of 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride (IPAN), 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), or their esters or acids, and selected aromatic diamines as interlevel dielectrics and coatings for on-chip and multi-chip packages in electronic applications and, more particularly, to multilayer structures for electronics use wherein low dielectric constant, low moisture uptake, thermally stable, high Tg polymers are required for use as protective coatings on substrates and interlevel dielectrics sandwiched between two layers.
Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant and high electrical resistivity. Such polymers have been used as passivation coatings, interlevel dielectrics, die attach adhesives, flexible circuit substrates, and more recently as the interlevel dielectric in high speed IC interconnections. Although current polyimides have the thermal stability necessary to meet the rigorous processing conditions required in on-chip and multi-chip packaging use, they do not exhibit the low dielectric constant and reduced moisture uptake shown by lower thermal stability polymers such as polyethylene and polytetrafluoroethylene. However, where thermal stability requirements are demanding, the latter polymers, despite their excellent dielectric and moisture uptake properties, cannot be substituted for polyimides.
In addition to those requirements stated above, a polymer for such use in the electronics industry must be resistant to processing solvents, form pinhole-free coatings when spin-coated, adhere well to a variety of substrates such as silicon dioxide and silicon nitride, aluminum, other polyimides etc., contain less than 1 ppm ionic impurities, exhibit appreciable etch rates in plasma etch or reactive ion etch, and have high electrical resistivity. Because of the multiple and severe requirements, new polymers with improved properties (particularly the combination of low dielectric constant, low moisture uptake, and thermal stability) are commercially significant and in great demand.
Low moisture uptake and good thermal stability are required for polymeric coatings useful in microelectronics applications. The set of polymer physical properties which must be optimized for interlevel dielectric use includes the latter two properties plus low dielectric constant. A low dielectric constant is useful in polymeric microelectronic coatings too, but the value of the polymer dielectric constant is particularly important in the interlevel dielectric use.
The use of polyimides in industry, particularly in the coating art, is not new, and a number of publications exist trading their preparation and details of their uses. However, the definition of and optimization of the relevant polymer properties, particularly in the use of polyimides for microelectronics, still struggles with defining the polymer properties desired and relating the desired properties to the chemical and physical nature of the polyimide.
In U.S. Pat. No. 4,681,928 to M&T Chemicals, Inc., a vast number of polyimides, polyamides, etc., said to be useful generally for coatings are made from diamine and diacid compounds. Included in the list is the dianhydride, IPAN, and the diamine, 4,4'-[1,4-phenylene-bis(1-methylethylidene)] bisaniline (BAP). Very limited exemplification is made.
In U.S. Pat. No. 4,720,539 to Rabilloud et al., a number of polyimides for "insulating coatings and films and for enameling electric or magnetic wires" (see Column 1, lines 12-13) are disclosed, but the teaching does not describe the low dielectric constant, low moisture uptake, high Tg, thermally stable combination of properties which are believed important for the present electronics uses. The polymer made from 2,2-bis(4-aminophenyl)propane (BAA) and 6FDA is taught in Rabilloud et al. as an insulating coating but no mention is made of the interlevel dielectric use.
U.S. Pat. No. 4,528,004 to UBE is directed to a process for making an aromatic polyimide composites in the form of a porous membrane which is useful for gas separation. Although some of the polyimides are the same as those taught herein, the set of physical properties useful for gas permeable membranes is not the same set useful in coatings and interlevel dielectrics for electronics uses. Porous membranes and coatings are different uses of polyimides.
U.S. Pat. No. 4,760,126 to Hitachi teaches the use of polyimides for moisture resistant, heat resistant and solvent resistant coatings useful for electronic devices. Hitachi is silent on the low dielectric constant property desired here. In addition, the polyimides taught are different than those of Applicants and are made from complex acid dianhydrides having oxygen, ester or thioester linkages which insure that the Hitachi polyimides have higher dielectric constants than those of Applicants. The dianhydrides taught herein are different, being derivatives of the &gt;C(CH.sub.3).sub.2 or &gt;C(CF.sub.3).sub.2 group, which lead to chemically distinct polyimides having potentially lower dielectric constants. Although "co-use" of the two acid dianhydrides, IPAN and 6FDA, along with many others is mentioned at column 6, lines 11-12 of the '126 patent, the polyimides taught and claimed by '126, even with the "co-use," are not those here found valuable. Some of the present diamines are taught in the '126 patent, but none of the Hitachi polyimides are those used in this invention.
In U.S. Pat. No. 4,336,175 to Dupont, laminates made from 6-FTA (6FDA) and aromatic diamines including some of those disclosed herein are taught. The teaching is to an improved way of making the polyimides which are then combined with a filler to make the laminate. Coatings and interlevel dielectrics for electronics use are not taught, and the physical properties of the polyimides are thus not optimized for use as coatings and interlevel dielectrics.
In Polym. Prepn. 29 (1) 349-51 (1988) and a report from the 33rd International SAMPE Symposium, March 7-10, 1988, a number of polyimides including one made from 6FDA and BAP were studied, and physical properties such as thermal stabilities as measured by TGA curves, dielectric constants, intrinsic viscosities (IVs), and solubilities are reported. The polymers are generally disclosed to be those with "improved dielectric behavior" and useful for "coating applications."
Polyimides useful for composites which are made from 6FDA and 4,4'-bis(p-aminophenoxy)biphenyl (APBP) are set forth in PCT Int. Appln. WO 86/4073 (1986) and in a paper by G. E. Chang and R. J. Jones, 28th Natl. SAMPE Symp., pp. 728-39 (1883). Polyimides and copolyimides made from APBP and either pyromellitic dianhydride or 3,3',4,4'-biphenyltetracarboxylic dianhydride are disclosed in a series of Japanese patents to Nitto Electric Ind., J.P. 61-143,433, 4 and 5, for use as electric insulating, heat- and wet-resistant polymers.
Now it has been found that improved polymers exhibiting most if not all of the above property requirements for microelectronic coatings and interlevel dielectrics can be found in aromatic polyimides by the careful selection of both the amine moiety and anhydride moiety used to make the polymers.